The immune system of vertebrates is characterized by its ability to discriminate “self” from “non-self” and to mount an appropriate, selective response to pathogens and other potentially harmful agents. Cell types involved in immune responses include lymphocytes known as B cells and T cells. Interactions between B cells and T cells are important for the propagation of full immune responses.
T cells must be activated in order to effect an immune response. T cell activation is thought to require two signals: an antigen-specific signal and a signal that is not antigen-specific. T cells can become activated by binding to B cells, particularly B cells which are themselves activated.
B cell-mediated activation of T cells is thought to be mediated, at least in part, by B7 proteins, which are expressed on the surface of B cells. B7 proteins are members of the immunoglobulin superfamily. They bind to activated T lymphocytes and provide regulatory signals for T lymphocyte cell growth and activation. (See, e.g., “Immunobiology—The Immune System in Health and Disease”, 1997, Third edition, chapter 7, Janeway, C. A. et al., eds., Garland Publishing Inc., New York.) Cell surface molecules on T cells which bind to B7 molecules include CTLA4 and CD28.